A urine C&S (culture and sensitivity) is a lab test that grows bacteria or fungi from a urine sample to diagnose a urinary tract infection and determine which antibiotics will treat it effectively. It goes beyond a standard urinalysis, which only screens for signs of infection like white blood cells. The culture identifies the exact organism causing the problem, and the sensitivity portion reveals which drugs that organism responds to.
How It Differs From a Urinalysis
A urinalysis is a quick screening test. It checks the appearance of your urine, measures chemical markers, and looks for blood cells or bacteria under a microscope. It can suggest an infection is likely, but it can’t tell your doctor exactly which bug is responsible or how to treat it.
A urine culture is a separate step. If your urinalysis raises a red flag, your provider sends the same or a new sample to the lab for culturing. There, technicians place the urine on growth plates and wait to see what grows. Cultures are not part of routine urinalysis. They’re ordered specifically when an infection needs to be confirmed and targeted with the right medication.
Why Your Doctor Orders One
The most common reason is suspected UTI. That means symptoms like burning during urination, an urgent or frequent need to pee, lower abdominal pressure, or cloudy and strong-smelling urine. Your provider might also order a C&S if you have an unexplained fever with no obvious source, or if a previous round of antibiotics didn’t clear your symptoms. Recurrent UTIs, complicated infections in people with catheters or structural abnormalities, and UTIs during pregnancy are other common triggers.
Collecting a Clean-Catch Sample
Most urine C&S tests use a “clean-catch midstream” sample. The goal is to avoid contaminating the specimen with skin bacteria, which can produce misleading results. If possible, collect the sample when urine has been sitting in your bladder for two to three hours.
Start by washing your hands. If you have a vagina, use sterile wipes to clean between the labia from front to back, then wipe over the urethral opening. If you have a penis, clean the head with a sterile wipe (pull back the foreskin first if uncircumcised). Begin urinating into the toilet, then stop midstream and hold the sterile cup a few inches from your body to catch the middle portion until the cup is about half full. Finish urinating into the toilet.
Screw the lid on tightly without touching the inside of the cup. If you’re collecting at home, place the sealed cup in a plastic bag, refrigerate it, and bring it to the lab as soon as possible.
When a Clean-Catch Isn’t Enough
In some situations, a clean-catch sample is unreliable. Infants and young children who aren’t toilet-trained are the clearest example. The American Academy of Pediatrics recommends catheterization or a needle-based collection called suprapubic aspiration for children aged 2 to 24 months with unexplained fevers. In one study of 599 children under two years old, contamination rates were 26% with clean-catch, 12% with catheterization, and just 1% with suprapubic aspiration. Adults may need catheterized or aspirated samples when a bladder outlet obstruction, urethral injury, or other anatomical issue makes a normal void impossible.
What Happens in the Lab
Lab technicians spread a measured amount of your urine onto growth plates, typically blood agar (a general-purpose medium) and MacConkey agar (which selectively grows the gram-negative bacteria responsible for most UTIs). If a fungal infection is suspected, they may add a specialized plate for yeast. The plates are incubated at body temperature, around 37°C (98.6°F), for 16 to 24 hours.
After incubation, technicians examine what grew. If bacteria or fungi appear, they identify the species. Then comes the sensitivity portion: the lab exposes the identified organism to a range of antibiotics at different concentrations. The lowest concentration that stops visible bacterial growth is called the minimum inhibitory concentration, or MIC. By comparing that number against established breakpoints, the lab classifies each antibiotic into one of three categories on your report.
Reading Your Results
Results typically come in two parts: the culture and the sensitivity panel.
The culture portion tells you whether anything grew and, if so, what organism was identified. Growth is measured in colony-forming units per milliliter (CFU/mL). The traditional threshold for a positive result is 100,000 CFU/mL or higher, which is the cutoff recommended by the American Society for Microbiology for midstream samples. However, some women with symptomatic UTIs have counts as low as 100 CFU/mL, and many labs now report growth at 1,000 CFU/mL or above. Context matters: a lower count in someone with clear UTI symptoms may still be clinically significant, while a high count in someone with no symptoms could simply reflect harmless colonization.
The sensitivity panel lists the antibiotics tested alongside each one’s classification:
- S (Susceptible): The organism is likely to respond to this antibiotic at standard doses.
- I (Susceptible, Increased Exposure): The antibiotic may work, but it might require a higher dose or a formulation that concentrates at the infection site.
- R (Resistant): The organism will not respond to this antibiotic, even at higher doses.
Your provider uses this panel to pick an antibiotic that’s classified as “S” for your specific infection, which is especially valuable if you’ve taken antibiotics recently or have a history of resistant infections.
Common Organisms Found
The single most common cause of UTIs, both uncomplicated and complicated, is a strain of E. coli adapted to the urinary tract. After E. coli, the organisms differ slightly depending on the type of infection. For straightforward UTIs, the next most frequent culprits are Klebsiella pneumoniae, Staphylococcus saprophyticus (particularly in young women), Enterococcus faecalis, group B Streptococcus, and Proteus mirabilis. For complicated UTIs (those involving catheters, structural abnormalities, or hospital-acquired infections), Enterococcus species and Candida yeast move higher up the list, along with Pseudomonas aeruginosa and Staphylococcus aureus.
Knowing the exact organism helps explain certain symptoms. Proteus, for instance, makes urine more alkaline and is associated with certain types of kidney stones. Candida infections are more common in people with diabetes or those who’ve been on prolonged antibiotics.
How Long Results Take
A negative culture, meaning nothing significant grew, can often be reported within 24 to 48 hours. A positive culture takes longer because the lab needs additional time to identify the organism and then run sensitivity testing. Expect preliminary results (which organism is growing) within two to three days, with the full sensitivity panel potentially taking an additional day or two. In total, final results usually arrive within three to five days from when the lab receives your sample.
Because of this delay, your provider may start you on a broad-spectrum antibiotic right away if your symptoms are severe or your urinalysis strongly suggests infection. Once the sensitivity results come back, they can switch you to a more targeted drug if needed.
When a Positive Culture Doesn’t Need Treatment
Bacteria in the urine without any symptoms is called asymptomatic bacteriuria, and it’s surprisingly common, especially in older adults, people with diabetes, and those with indwelling catheters. In most of these cases, treating with antibiotics does more harm than good: it increases the risk of side effects and antibiotic resistance without improving outcomes. The major exception is pregnancy, where untreated bacteriuria raises the risk of kidney infection and complications for the baby, so screening and treatment are standard during prenatal care.